1. Field of the Invention
The present invention relates generally to the cleaning of a microelectronic workpiece. More particularly, it concerns an improved method and apparatus to remove submicron-sized contaminants from the surface of a workpiece using megasonic energy.
2. Description of Related Art
In general, at one or more stages in the semiconductor fabrication process, there is a need to clean the wafer or other microelectric workpiece to remove contaminants and other residue. Contaminants typically include films, discrete particles, particulates, micro-droplets, vapors and residue.
As the design rule of semiconductor devices decreases to the submicron region, the tolerable size of the contaminants produced during the fabrication process is also decreased. Even extremely small foreign particles may cause fatal defects during the fabricating process, thereby decreasing the reliability and yield of the fabrication facility. As such, cleaning a workpiece to remove particulates and residues down to 0.05 micrometers is important.
These submicron-sized particles are difficult to displace because of strong adhesion forces between the particles and the substrate, such as caused by Van der Waals forces, capillary forces, chemical bonding, and/or electrostatic forces.
Early cleaning techniques used brushes to physically sweep the particles from the surface. However, with the increased level of miniaturization, many substrates are delicate and can no longer be cleaned with brushes. Brushes are abrasive, and the metalization on wafers and masks cannot tolerate such abrasive cleaning processes.
Other prior art systems use high-pressure water jets to direct a liquid stream incident on a rotating wafer to remove contaminants. Often, however, the high pressure within the water column causes damage due to the large forces they exert, particularly to metal patterns on the wafers. Additionally, high-pressure water jets typically require large quantities of deionized water, which may not be economical for a manufacturing environment.
Other cleaning methods include submerging the semiconductor part in a tank having transducers attached to the bottom for applying acoustic energy. The acoustic energy is either in ultrasonic or megasonic frequencies. Ultrasonic cleaning causes cavitation effects, and therefore results in micro explosions that are undesirable for today""s microchip manufacturing. As such, megasonic energy has emerged as a preferred way to remove contaminants from a semiconductor parts because it does not cause cavitation. An additional advantage from using a megasonic tank is the ability to clean the top and bottom surfaces of an article simultaneously. However, removing cleaned articles from a tank is problematic because particles frequently reattach to the surface as the article is being removed. This reattachment is possible even when using recirculating tanks with filtering. Another problem is that the tank itself is an acoustical cavity, with standing waves, peaks, and valleys. Further, the energy coupled to the tank is affected by the loading of the tank. Because of both of these problems, the applied megasonic energy density varies between points on a wafer, between wafers within the same tank, and between tanks. Additionally, energy density is limited due to the fact that all energy has to distribute to the full volume of the tank. For example, typical megasonic tanks with megasonic transducers mounted to the bottom of the tank inherently produce an energy density gradient between the top and bottom of the tank. An additional drawback to cleaning semiconductor parts in a tank is the large quantities of cleaning fluids required. These chemicals are expensive and generally hazardous to the environment.
Some prior art systems began incorporating megasonic nozzles in conjunction with other techniques where acoustic energy is transferred to a wafer through a deionized water stream, in an attempt to retain the benefits of megasonic cleaning without using a tank. For example, U.S. Pat. Nos. 5,368,054 and 5,562,778 describe the use of megasonic nozzles to apply energy through a cleaning stream to remove particles from a substrate. Generally, the article such as a wafer or substrate was rotated or moved linearly under the water stream to apply megasonic energy to a total surface area. However, rotation of the wafer and movement of the water stream were generally done without attention to the total energy delivered to each point on the substrate. Therefore, energy density gradients and a corresponding variation in the cleanliness of points on a substrate exist.
U.S. Pat. No. 5,980,647 describes a megasonic nozzle system that mentions the uniform application of megasonic energy. However, during cleaning, the system in the ""647 patent rotates the substrate at high RPMs. Use of high-RPM cleaning has a number of disadvantages including inherent non-uniformity of the energy delivered (particularly at low radial lengths), and drying or partial drying of the substrate during the cleaning process. In addition, in the system described in the ""647 patent, a nozzle directs the stream of cleaning fluid perpendicular to the surface to be cleaned, which can lead to the stagnation of cleaning fluid on the substrate. Furthermore, the ""647 patent does not address the drying the workpiece, which is important to successfully removing contaminants and keeping them from reattaching to the substrate.
Without proper drying, wafers are generally not cleaned successfully. Even in a state-of-the-art clean room a wet substrate is capable of attracting contaminants. Further, if a substrate is subjected to contaminants before drying, contaminants may be bonded to the surface. Therefore, drying a semiconductor part to effectively remove all cleaning fluids before contaminants have the opportunity to reattach is an important step in the cleaning process. Many prior art systems use separate machines for cleaning and drying. Thus, as the workpiece is cleaned, it is removed from the cleaning apparatus and placed into another machine for drying. During the time the workpiece is removed from the cleaning device, transported to the drying device, and placed in the drying device, there is a probability of contamination and of removed particles reattaching.
In light of the shortcomings of the prior art, it would therefore be desirable to have an apparatus capable of effectively cleaning a semiconductor part and drying the semiconductor part using the same apparatus. It would further be advantageous that if on the same apparatus, chemicals (acids, solvents, surfactants, etc.) could be dispensed for chemical cleaning of hard to remove contaminants or modify pH of the cleaning solution to reduce the electrostatic forces between particles and the substrate prior to loosening or detaching with megasonic energy and then removing debris and drying the surface with a centrifugal force. Such a device may also integrate chemical processing (etching) and cleaning into a single process further reducing the process steps and defects. In addition, other processes such as the detachment process can be carried with the aid of megasonic agitation and then be followed with a complete cleaning. It would further be desirable for using the megasonic energy to passify newly cleaned surfaces by producing very small amounts of Hydrogen that could attach to surface. Other chemicals and reactants, for example hydrogen peroxide, can be formed on the surface with or without the aid of megasonic energy for the improved removal of foreign materials and followed by megasonic cleaning, surface passivation, and spot free drying to achieve better yields.
In one broad respect, the present invention is directed to a method for removing contaminants from a workpiece comprising the steps of: rotating the workpiece about an axis at a low angular velocity; directing a stream of cleaning fluid at a surface of the workpiece, such that the stream of cleaning fluid delivers substantially uniform megasonic energy to all points on the surface of the workpiece, and then rotating the workpiece at a high angular velocity until the surface of the workpiece is dry of cleaning fluids. In a narrow respect, a low angular velocity is between 200 and 500 revolutions per minute. In another narrow respect, a high angular velocity is 3500 revolutions per minute or higher. In another narrow respect, the cleaning fluid comprises deionized water. In a narrower respect, the deionized water is ozonated. In another narrow respect, the cleaning fluid is an acid. In another narrow respect, the cleaning fluid includes one or more surfactants. In another narrow respect, the cleaning fluid is operable to alter the Zeta potential of one or more of the contaminants and the surface of the workpiece to increase the repulsion between the one or more contaminants and the workpiece. In a narrower respect, the method further comprises the step of delivering a cleaning fluid to the workpiece while the workpiece is rotating at a low angular velocity. In another narrower respect, the method further comprises the step of delivering drying fluids to the workpiece while the workpiece is rotating at a high angular velocity. In a narrower respect, the workpiece is a semiconductor wafer.
In another broad respect, the present invention is directed to a method for removing contaminants from the surface of a workpiece, comprising the steps of: rotating the workpiece bout an axis, at a low angular velocity; directing a stream of cleaning fluid, wherein the stream of cleaning fluid comprises megasonic energy, through a nozzle at a surface of the workpiece, while moving the nozzle over the workpiece such that the stream of cleaning fluid delivers substantially uniform megasonic energy to all points on the surface of the workpiece; and then rotating the workpiece at a high angular velocity until the surface of the workpiece is dry of cleaning fluids. In a narrow respect, the step of moving the nozzle over the workpiece further comprises moving the nozzle over the workpiece in a stepwise manner. In another narrow respect, the step of moving the nozzle over the workpiece in a stepwise manner comprises stopping the nozzle over each radial strip for an amount of time proportional to the radial distance of the strip. In another narrow respect, the step of directing a stream of cleaning fluid further comprises directing the stream of cleaning fluid at an optimal angle to the surface of the workpiece. In a narrower respect, the optimal angle is between 60 and 70 degrees with respect to the workpiece surface. In another narrow respect, the nozzle is maintained at a fixed distance from the surface of the workpiece during the step of moving the nozzle over the workpiece. In a narrower respect, the fixed distance is approximately 12 mm.
In another broad respect, the present invention is directed to a method for removing contaminants from the surface of a workpiece having a top surface and a bottom surface, comprising the steps of: rotating the workpiece at a low angular velocity; directing a first stream of cleaning fluid, wherein the first stream of cleaning fluid comprises megasonic energy while moving the first nozzle over the top surface such that the first stream of cleaning fluid delivers substantially uniform megasonic energy to all points on the top surface, wherein at least a portion of the megasonic energy delivered to the top surface propagates through to the bottom surface; directing a second stream of cleaning fluid at the bottom surface of the workpiece; and then rotating the workpiece at a high angular velocity until the workpiece is dry of cleaning fluids. In a narrow respect, the second stream of cleaning fluid comprises deionized water.
In another broad respect, the present invention is directed to a method for cleaning contaminants from the surface of a workpiece, comprising the steps of: applying megasonic energy to at least one stream of cleaning fluid, hereinafter the megasonic cleaning stream; directing at an optimal angle the at least one megasonic cleaning stream to the surface of the workpiece; rotating the workpiece at a low angular velocity sufficient to maintain a uniform laminar flow profile of the megasonic cleaning stream on the surface of the workpiece; and moving the megasonic cleaning stream over the workpiece such that a substantially uniform amount of megasonic energy is delivered to each point on the workpiece. In a narrower respect, the method further comprises the step of rotating the workpiece at a high angular velocity until the workpiece is dry of cleaning fluids.
In another broad respect, the invention is directed to a system for the effective removal of contaminants from the surface of a workpiece, comprising: a chuck operable to hold the workpiece; a spinner motor operable to rotate the chuck about an axis; a fluid source containing a cleaning fluid; a nozzle coupled to the fluid source, wherein the nozzle is operable to apply megasonic energy to the fluid source, and wherein the nozzle is further operable to direct a stream of fluid energized with megasonic energy towards the surface of the workpiece held by the chuck; an arm coupled to the megasonic nozzle, the arm capable of positioning the nozzle over the surface of the workpiece held by the chuck; an arm motor for positioning said arm, wherein the arm motor is a stepping motor; and one or more controllers operable to control the nozzle, the spinner motor, and the arm motor, such that: the nozzle directs the stream of cleaning fluid towards the surface workpiece held in the chuck, the spinner motor rotates the chuck at a low angular velocity, and the arm motor moves the arm and the nozzle transversely over the surface of the workpiece in a step-wise manner such that the stream of cleaning fluid delivers substantially uniform megasonic energy to all points on the surface of the workpiece. In a narrow respect, the low angular velocity is between 200 and 500 revolutions per minute. In another narrow respect, the one or more controllers is further operable to stop the nozzle from directing the stream of cleaning fluid towards the surface of the workpiece, and to rotate the chuck at high angular velocity to dry the workpiece of cleaning fluids. In another narrow respect, the high angular velocity is 3500 revolutions per minute or higher. In another narrow respect, the system further comprises a conduit coupled to the fluid source, wherein the chuck is capable of holding the workpiece in a manner that exposes substantially all of the bottom surface of the workpiece to be cleaned, and wherein the conduit is operable to direct a stream of cleaning fluid to the bottom surface of the workpiece. In another narrow respect, the system further comprises a second fluid source, the second fluid source containing a cleaning fluid, a conduit coupled to the second fluid source, wherein the second nozzle is operable to direct a second stream of cleaning fluid onto the surface of the workpiece. In another narrow respect, moving in a step-wise manner comprises stopping for a controlled length of time proportional to the radial position of the nozzle.
All references incorporated herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated by herein, it is incorporated by reference for background purposes, and indicative of the knowledge of one of ordinary skill in the art.